1. Field of the Invention
The present invention relates generally to electromagnetic actuators and, in one example of the invention, to positioning systems and methods, and more specifically to wafer positioning systems and methods which control the movement of a substrate such as a semiconductor wafer on which numerous integrated circuits are located.
2. Background Information
Wafer positioning systems are used in different areas of semiconductor fabrication, such as wafer probing and photolithography. Most wafer positioning systems are based on combinations of linear motions of one of more stages in orthogonal directions.
One such wafer positioning system, as shown in FIGS. 1A-1B and 2, uses linear motors, such as Sawyer motors. Examples of such wafer positioning systems are manufactured by Electroglas of Santa Clara, Calif. The linear motors on wafer holding stage 10 are outboard motors and inboard motors bonded to a material layer 18, which can be made of aluminum. Each outboard motor comprises a pair of electromagnetic assemblies 20 coupled to each other by a permanent magnet 21. The surfaces of electromagnetic assemblies 20 have an alternating pattern of positioning teeth 32 and grooves 34. Each inboard motor comprises a pair of electromagnetic assemblies 24 coupled to each other by a permanent magnet 25. The surfaces of electromagnetic assemblies 24 have an alternating pattern of positioning teeth 28 and grooves 30. Grooves 30 and 34 are typically filled with epoxy to provide level surfaces. Positioning teeth 28 and 32 are magnetizable regions, while grooves 30 and 34 are nonmagnetizable regions.
Wafer holding stage 10 typically rides on the surface of a base stage 12 with the help of air bearings (not shown). Intersecting grooves 16 are etched into the surface of base stage 12 to from a repeating pattern of positioning teeth 14. Grooves 16 are typically filled with epoxy to provide a level surface on base stage 12. Positioning teeth 28 and 32 on wafer holding stage 10 can match up with a line of positioning teeth 14 on base stage 12 as the flux of the permanent magnets of the inboard and outboard motors is diverted among the poles of the electromagnetic assemblies. As the flux is diverted and different lines of teeth match up, wafer holding stage 10 moves incrementally across the surface of base stage 12.
An important feature of a base stage such as base stage 12 is the uniformity of the pattern of teeth, because the uniformity has a direct effect on the accuracy of the incremental movements of the wafer holding stage. Because the teeth are formed by etching intersecting grooves in the surface of the base stage, the etching of the grooves takes on great importance. Groove dimensions such as depth and width are critical in determining teeth uniformity. Unfortunately, current groove etching processes do not adequately provide the level of consistency to create grooves and teeth that are necessary to meet the demands of increasing stage movement accuracy.
Another important feature of a base stage is the size of the teeth. With smaller teeth, flux is concentrated into smaller areas, which means saturation occurs more quickly. With larger teeth, the saturation threshold is higher, thereby allowing more flux to flow through the teeth, which means a greater amount of force is available to move the wafer holding stage. Increased force provides increased speed. Unfortunately, narrow grooves, and hence larger teeth, are difficult to create using current techniques.